Responsa for Bava Batra 317:4

ומאי קושיא דלמא מצי אמר מכח אבוה דאבא קאתינא ובמקום אב קאימנא

'A firstborn son who sold the share of [his] birthright during the lifetime of his father, and he died during the lifetime of his father, his son may take [it] away from the buyers; and this it is that presents a difficulty in civil law', [for] his father sold [it] and he takes [it] away! And if it be suggested [that] in this case<span class="x" onmousemove="('comment',' Lit., 'here'. ');">¹³</span> also [he might plead]. 'I come as successor to the rights<span class="x" onmousemove="('comment',' Lit., 'from the power'. ');">¹⁴</span> of my father's father', [it may be retorted.] 'If he comes as successor to the rights of his father's father what claim has he upon the portion of the birthright?'<span class="x" onmousemove="('comment',' Were it not for the rights of his father who was a firstborn son, he should not have been entitled to the double portion! ');">¹⁵</span> But what difficulty [is this]? Could he not<span class="x" onmousemove="('comment',' Lit., 'perhaps'. ');">¹⁶</span>

Teshuvot Maharam

Teshuvot Maharam